1. Field of the Invention
This invention pertains to acoustic wave devices and more particularly relates to surface acoustic wave devices and shallow bulk acoustic wave devices wherein the use of separated active and base substrates permits low aging rates and low acoustic losses to be obtained.
2. Description of the Prior Art
Narrow band acoustic wave devices including surface acoustic wave (SAW) devices and shallow bulk acoustic wave (SBAW) devices must have stable long term response characteristics in order to be of general utility. Conventional SAW and SBAW devices are fabricated by placing an electro-acoustic transducer on or recessed into an active piezoelectric substrate such as quartz (SiO.sub.2) on or in which propagation of the acoustic wave occurs. The electro-acoustic transducer electrodes are normally fabricated from a metal such as aluminum with a thin chrome layer for adhesion to the quartz substrate. Gold electrodes have also been used, however, gold provides a poor acoustic match to quartz thereby making acceptable response characteristics difficult to achieve.
The deposit of the metal electrodes directly onto a quartz substrate is a major source of device aging over extended periods of time. Such device aging occurs from interactions between the metal and the substrate and include stress relief and chemical changes.
It is well known in the art that stressing the acoustic substrate alters the substrate's properties, such as the elastic constants, dielectric constants, piezoelectric constants and consequently the surface acoustic wave velocity. As a result, the device's operating frequency may be undesirably affected. Thin adherent metal films are ordinarily under stress when disposited. Ordinarily the thin metal films that comprise a transducer's electrodes are deposited by techniques such as evaporation; consequently, as the metal cools it will experience thermal contractions. Since the metal adheres to the substrate, it tends to stress the substrate as it contracts. Over a period of time, depending on the degree of ductility the metal possesses, the metal will be stretched and the stress placed upon the substrate will be relaxed. The mechanical properties of the surface of the acoustic substrate may thus change and as a result, the acoustic wave device's operating characteristics may also change. Additionally, micro-cracks which may be present in the substrate material during fabrication may further propagate throughout the crystal substrate due to strain on the substrate. These micro-cracks may also change the acoustic wave device's characteristics, and possibly lead ultimately to failure of the device. Chemical changes will also change the characteristics of the substrate. Such chemical changes include the formation of metal oxides and reduction of the quartz to silicon in the vicinity of the metallized electrodes.
Changes in metal films will also occur due to electromigration of the metal and stress induced metal migration. Actual migration of metal atoms along grain boundaries in the direction of an electric field have been observed and similar metal migration has been observed in the presence of dynamic stresses. All of the above changes cause variations in the acoustic propagation path with a resultant change in device operating frequency and frequency response.
Prior art methods for reducing such aging effects have included the use of different metals. Gold is one metal which has been used in an attempt to overcome aging effects. Since gold is considerably more ductile than metals such as chromium or aluminum, aging due to stress relief is decreased. Furthermore, since gold is more inert than aluminum, chemical reduction of the quartz substrate and other chemically induced effects are minimized. Gold, however, as stated previously, provides a poor acoustic match to quartz and for this reason does not provide a completely satisfactory solution to the problem of aging.
Acoustic devices such as SBAW filters and SAW devices suffer appreciable acoustic losses as a wave travels from the electroded area to the free surface of the substrate due to the mass loading effect of the transducers. Furthermore, in SBAW filters in which acoustic waves are launched at an angle slightly depressed from the surface of the substrate, acoustic losses can be even more severe due to end effects of the transducers. In both SAW and SBAW devices such acoustic losses which result from the placement of the transducer on or in the surface of the active substrate cause a diminution of available signal energy and are hence undesirable.
The present invention provides acoustic wave devices in which aging effects due to the metallization of the active substrate and acoustic losses due to the metallization of the active substrate are eliminated using fabrication techniques adaptable to those in the known art.